Existing infra red (IR) detection systems generally include an IR detector (e.g. focal plane array (FPA)) and a dewar assembly enclosing the field of view of the detection systems and operating as a thermal shield masking the detector from thermal radiation arriving out of the enclosed field of view.
Many types of IR detectors operate properly when cooled to very low temperatures (e.g. of the order of 77 to 100° K) and accordingly in IR detection systems of this type the detector is thermally coupled with (e.g. mounted on) a cooling system. Other types of IR detectors, referred to herein as un-cooled IR detectors (e.g. bolometric IR detectors), can operate in ambient temperature conditions and may thus be used without being coupled to a cryogenic cooling system.
Both the cooled and the un-cooled IR detector types are sensitive to radiation arriving within a wide field of view angle (of about 180°) with respect to certain optical axis typically perpendicular to the sensitive area of the detector. In this respect, in order to limit the field of view of the detector to a certain desired field of view of detection system, optical/thermal shields are generally used to prevent radiation, coming from out of the desired field of view, from reaching the detector.
In this regards, one of the functions of dewar assemblies is to limit the effective field of view of the IR detection system to a desired field of view defined by a certain solid angle of light propagation about the optical axis of the detector (smaller than the wide field of view angle of the IR detector itself). Accordingly, dewar assemblies are typically configured to enclose the IR detector with a cylindrical-like structure that at least partially surrounds the optical axis of the IR detection system and defines an optical window (e.g. an optical aperture thereof) through which radiation within the desired field of view reaches the IR detector.
To this end, typical dewar assemblies include cold and warm shields associated respectively with a cold filter and a warm window which are mounted thereon. The cold shield serves to at least partially circumference the optical axis of the IR detection system and to thereby allow radiation of only a limited field of view to arrive at the IR detector. Since the cold shield may by itself emit thermal radiation (parasitic radiation), the cold shield is maintained cooled to relatively low temperatures (e.g. about 100° K) such as to reduce emissions of such parasitic thermal radiation from the shield. Therefore, cold shields are typically enclosed within a warm shield which provides thermal isolation of the cold shield from the outside temperature.
Since the cold shield encloses a portion of the optical path (typically between the cold filter and the detector), it may by itself reflect radiation towards the IR detector, which is undesired radiation entering the system the through the cold filter and impinging onto the walls of the cold shield. Reducing the amount of out of field of view radiation reflected on the detector from the walls of the cold shield can be achieved by utilizing highly emissive/absorptive (blackened) inner surfaces of the cold shield to increase the absorption of such unwanted radiation. It is also known to configure the inner surfaces of the cold shield to be reflective and having special geometrical shapes designed to reflect undesired radiation out of the detector's field of view. The cold shield inner surfaces are cooled for reducing the thermal radiation emitted therefrom.
There are many known techniques aimed at increasing the emissivity of cold shields. According to some of these techniques, a cold shield includes several baffles to support the reduction of unwanted IR radiation. An example of such technique utilizing baffle structure of the cold shield is described in U.S. Pat. No. 5,225,931 according to which an optical system which is provided with a tube having an open front end and a back end, where imaging optics is mounted in the tube, and a plurality of light reflective baffle portions are provided rotationally symmetric with respect to the optical axis. The first baffle portions are configured as ellipsoids of revolution, all with foci lying adjacent edge portions of the open front end of the tube and facing the open front end. The second baffle portions are configured as hyperboloids of revolution facing away from the open end and inwardly of the tube.
U.S. Pat. No. 4,820,923 discloses a warm shield reflector for a cryogenically cooled radiation detector having a reflective surface of toroidal shape. The surface has geometric properties which cause a ray emanating from the detector to be reflected such that a ray is imaged as a defocused ring outside of and surrounding the active detector area. Several such segments are located in front of a small, cryogenically cooled detector shield, to provide an overall detector shielding effect similar to that of a larger, cryogenically cooled shield.
U.S. Patent Publication 2006/180765 describes an infrared imaging system that uses an uncooled elliptical surface section between reflective surfaces to allow a detector to perceive a cold interior of a vacuum chamber rather than a warmer surface of a structure or housing. In this way, background infrared radiation from within the system may be minimized.
WO 07/003729 describes an electromagnetic radiation detection device consisting of a sensor having a surface that is sensitive to said radiation and a cold shield comprising a lateral wall having a cross-section that forms an elliptical arc profile, such that no ray reflected by the wall, originating from an incident ray, comes into contact with the sensitive surface of the sensor.